The breakdown voltage of bipolar devices such as NPN BJTs is rather high. In order to achieve the appropriate breakdown voltage in such devices so as to provide ESD protection in the desired voltage domain one prior art solution has been to provide a reference current at a certain reference voltage by introducing a voltage reference device into the base-collector circuit of the bipolar device, thereby providing what is also referred to as an enhanced Zener clamp in the case of a Zener diode as reference device. At a reference voltage that exceeds the breakdown voltage of the reference device (whether it be a Zener or a diode or stacked diode arrangement), base current is produced which triggers the NPN BJT or BSCR ESD device.
However, this prior art solution requires not only extensive experimentation with the breakdown characteristics of the reference device, it also requires additional space on the chip to accommodate the reference device. A schematic representation of a BJT 100 with a Zener reference device 102 is shown in FIG. 1. The Zener 102 is connected between the I/O pad or Vdd and the base of the BJT 100, the current path being completed by a resistor 104 connected between the base and ground. The arrangement of the bipolar device 100, 300, resistor 104, 304, and reference Zener 102 is shown in simplified form in FIG. 2, which shows an NPN BJT 100 arranged in an enhanced Zener clamp arrangement. The same reference numerals were retained for FIG. 2 as for FIG. 1 to identify the corresponding structures. FIG. 3 shows the layout for a BSCR 300 making use of a stacked diode arrangement 302 to provide the reference voltage, and also requiring a resistor 304 to complete the current path. It will therefore be appreciated that the additional reference device, whether it be a Zener or a diode or stacked diode arrangement, and the resistor consume a considerable amount of chip space.